Radio frequency (RF) based systems are used for communicating with various types of access controls, logging systems, cashless payment systems as well as transferring data between units such as pictures between cameras in cell phones. The basis of these systems is the RF electronics unit and an antenna. The antenna generates a signal for communication with the card or other device. This antenna is typically a few inches on a side. Because of the small size relative to the operating frequency, these antennas are tuned with a matching network and the coil is a small number of turns of wire or turns on a printed circuit board.
A problem with using these antennas is that they need to be kept away from metal. Typical recommendations are that the antennas be kept at least four inches from a metal surface. Unfortunately, this is not practical for many systems and an alternate way of dealing with metal is needed. This is the object of the current invention.
There are ways of overcoming mounting issues which fall into the categories of nonmetal housings and ferrite shields. Some of the deficiencies of these approaches are that security requirements may make it difficult to employ them. A plastic housing and antenna mount is vulnerable to an attack which would render it useless. There is a similar problem with ferrite shields. The ferrite shields are placed behind the antenna and in front of the metal mounting surface. This then requires an additional nonmetallic cover over the antenna and it has the same issues as for the plastic cover. These limits may not be an issue in, for example, cell phone applications where the back cover of the phone protects the antenna but it is an issue in industrial and exterior applications.
The deleterious effect of the metal is that it carries electrical current generated from the antenna. This current loads the antenna and reduces or stops the radiation of the desired signal which needs to get to the device to be communicated with. The mechanism employed by the ferrite shielding is to divert the RF field away from the metal and thereby eliminate the generated currents in the metal. Since it is desirable to have the antenna in the plane of the metal surface or below it, an alternate approach is needed.